The present invention relates to optics and, more particularly, to a device and method for optical resizing or backlighting.
Miniaturization of electronic devices has always been a continuing objective in the field of electronics. Electronic devices are often equipped with some form of display, which is visible to a user. As these devices reduce in size, their display size is reduced too. However, beyond some size the electronic device's display cannot be viewed with a naked eye and its image should be magnified.
An electronic display may provide a real image, the size of which is determined by the physical size of the display device, or a virtual image, the size of which may extend the dimensions of the display device.
Magnification of images produced by small size image display systems can be performed by projecting the image on a larger screen or via passive optical magnification element providing the user with a magnified virtual image. A virtual image is defined as an image, which cannot be projected onto a viewing surface, since no light ray connects the image and an observer.
It is appreciated, however, that the above magnification techniques are far from being optimal. Projected real images suffer from bulkiness since in projection the expansion of image is achieved by light propagation perpendicularly to the display. Devices producing virtual images have a limited field-of-view and are oftentimes also bulky.
In another magnification technique the image is not projected but rather guided through a bundle of optical fibers extending from a small facet to a large facet. The small facet is oftentimes referred to as “the object plane” whereas the larger facet is oftentimes referred to as “the image plane”.
Referring now to the drawings, FIGS. 1-2 are schematic illustrations of several prior art techniques for manufacturing fiber base guided magnifiers.
FIG. 1a shows an optical image transporting device based on the teachings of U.S. Pat. No. 2,825,260. The magnification from the small facet to the large facet is achieved by increasing the separation between the fibers in the bundle. FIG. 1b illustrates a modification to this method, disclosed in U.S. Pat. Nos. 2,992,587 and 3,853,658. In this technique, the fibers are up-tapered towards the large facet. These techniques, however, were not producible, due to the technological limitations associated with separation and up-tapering of optical fibers.
An attempt to overcome the up-tapering problem is disclosed in U.S. Pat. No. 3,909,109, where an additional layer is added at the large facet. The thickness of the layer is selected such as to allow free propagation through the layer until the far field beams of the fibers overlap. This technique, however, suffers from a major limitation, because the Gaussian shape of the far field line makes it difficult to determine the optimal thickness of the additional layer.
FIG. 1c shows another improvement of the device of FIG. 1a which improvement is based on the teachings of U.S. Pat. Nos. 3,043,910 and 4,208,096. In this improvement, the fiber separation is performed only in one dimension whereby the separation in the other (substantially orthogonal) dimension is done via by a terrace or slanted cut. In this configuration the fibers, after being separated in one direction, are redirected towards a large facet where they are terrace or slanted cut in order to be separated in the substantially orthogonal direction. A major limitation of this solution is the manufacturing difficulty.
FIGS. 2a-b shows another technique for producing a fiber optic magnification element, according to the teachings of U.S. Pat. Nos. 3,402,000 and 6,326,939. With reference to FIG. 2a, a one-dimensional magnification element includes cylindrically shaped optical fibers which are cut in a manner such that a circular cross section is formed on one side and an elliptic cross section is formed on the other side. The circular cross section is perpendicular to the longitudinal axis of the cylinder, and therefore has the same diameter as the cylinder. The elliptic cross section is slanted with respect to the longitudinal axis, hence has a small axis which equals the diameter of the cylinder and a large axis which is larger than the diameter of the cylinder. When light is transmitted through the fibers from the circular side to the elliptic side, a one dimension magnification is established in the direction of the large axis of the elliptic cross section.
With reference to FIG. 2b, two such one-dimensional magnification elements are connected via a redirecting layer such that the output of one element is used as the input of the other element. A second redirecting layer is used for coupling the light out of the second magnification element. To achieve proper optical coupling between the first and second elements, the cross section of the fibers on the input side of the second element must have the same elliptic cross section of the fibers on the output side of the first element.
However, the elliptic input cross section of the second element's fibers cannot be obtained by slanted cut because the input cross section of the fibers must be perpendicular to their longitudinal axis. On the other hand, a fiber bundle with elliptically shaped fibers does not exist. Therefore, in order not to loose resolution at the second magnification, the number of fibers in the second element should be larger than the number of fibers in the first element, by a factor which equals the one dimensional magnification ratio of the first element. Additional drawbacks of this technique are the need for redirecting layers and the presence of non-guided light which can diminish the display aspect ratio.
U.S. Pat. Nos. 5,511,141 and 5,600,751 disclose a reading magnifier formed by a bundle of juxtaposed longitudinally tapered optical fibers. The magnifier is commercially available under the trade name TaperMag™ from Taper Vision Co. Ltd., USA [E. Peli, W. P. Siegmund “Fiber-optic reading magnifiers for the visually impaired,” J Opt Soc Am A 12(10): 2274-2285, 1995]. The TaperMag™, however, is bulky (thickness of about 5 cm for only ×2 magnification up to a 2 inches screen) because its thickness must be comparable to the size of facet diameter.
U.S. Pat. No. 6,480,345 to Kawashima et al. discloses a magnifier which utilizes high-refractive-index regions extending from the small facet to the large facet. In simulations performed by Kawashima et al. it was found that a 30 inches magnifier can have a thickness of less than 4 cm and perform ten times enlargement. The manufacturing process of Kawashima's magnifier is, however, rather complicated. For example, one embodiment of Kawashima et al. involves the alignment of dozens of laminated thin plates produced by masks with increasing core dimensions. Another embodiment of Kawashima et al. involves three dimensions fiber handling. Although Kawashima et al. also teach simpler manufacturing processes, these are limited to magnification ratio of 2 or less.
Beside the magnification of the displayed images, efforts have been made over the years to research and develop display technologies for improving the quality of the images while reducing the power consumption and bulkiness of the display devices.
Generally, electronic display devices may be categorized into active display devices and passive display devices. The active display devices include the cathode ray tube (CRT), the plasma display panel (PDP) and the electroluminescent display (ELD). The passive display devices include liquid crystal display (LCD), the electrochemical display (ECD) and the electrophoretic image display (EPID). In active display devices, each pixel radiates light independently. Passive display devices, on the other hand, do not produce light within the pixel and the pixel is only able to block light.
Of the above display technologies, the passive display device, and in particular the LCD device has become the leading technology due to its proven high quality and small form factor (slimness). LCD devices are currently employed in many applications (cellular phones, personal acceptance devices, desktop monitors, portable computers, television displays, etc.), and there is a growing attention to devise backlight high-quality assemblies for improving the image quality inn these applications.
In LCD devices, an electric field is applied to liquid crystal molecules, and an alignment of the liquid crystal molecule is changed depending on the electric field, to thereby change optical properties of the liquid crystal, such as double refraction, optical rotatory power, dichroism, light scattering, etc. Since LCD are passive, they display images by reflecting external light transmitted through an LCD panel or by using the light emitted from a light source, e.g., a backlight assembly, disposed behind the LCD panel.
Backlight assemblies are designed to achieve many goals, including high brightness, large area coverage, uniform luminance throughout the illuminated area, controlled viewing angle, small thickness, low weight, low power consumption and low cost.
FIG. 42a illustrates a typical LCD device. The device includes a LCD panel and backlight assembly. The LCD panel includes an arrangement of LCD pixels, which are typically formed of thin film transistors fabricated on a transparent substrate with liquid crystal sandwiched between them and the color filters. The color filters which are fabricated on another transparent substrate produce colored light by transmitting only one third of the light produced by each pixel. Thus, each LCD pixels is composed of three sub-pixels. The thin film transistors are addressed by gate lines to perform display operation by way of the signals applied thereto through display signal lines. The signals charge the liquid crystal layer in the vicinity of the respective thin film transistors to effect a local change in optical properties of the liquid crystal layer.
In operation, the backlight assembly produces white illumination directed toward the liquid crystal pixels. The optical properties of the liquid crystal layer are locally modulated by the thin film transistors to create a light intensity modulation across the area of the display. Specifically, a static polarizer polarizes the light produced by the backlight assembly, and the liquid crystal pixels selectively manipulate the polarization of the light passing therethrough. The light intensity modulation is achieved using a static polarizer positioned in front of the liquid crystal pixels which prevents transmission of light of certain polarization. The color filters colorize the intensity-modulated light emitted by the pixels to produce a color output. By selective opacity modulation of neighboring pixels of the three color components, selected intensities of the three component colors are blended together to selectively control color light output. Selective the blending of three primary colors such as red, green, and blue (RGB) can generally produce a full range of colors suitable for color display purposes.
Traditionally, Cold Cathode Fluorescent tubes Light (CCFL) has been employed for LCD backlighting. A fluorescent lamps and optics are deployed for homogenously scattering the light across the LCD panel and color filters are deployed for separating between the colors. A diffuser layer and a reflector are used for further homogenizing the backlight spectrum and reducing optical leakage, respectively. To assure sufficient light transmission, color filters of relatively wide spectrum are used. This, however, results in crosstalks between the RGB pixels, which limit the available color gamut that can be obtained from CCFL backlighting. In addition, CCFL backlighting systems are expensive, bulky, power consuming and contain Hg.
In more advanced technique, a backlight assembly of LCD includes an array of Light Emitting Diodes (LEDs) for emitting white or RGB light, a light guiding plate for guiding the light toward the LCD panel, and a diffuser layer positioned between the LCD panel and the LEDs for homogenizing the backlight spectrum at the LCD panel. Oftentimes, a reflector is disposed behind the light guiding plate to reflect the lights leaked from the light guiding plate toward the light guiding plate. The LEDs, due to their inherent narrow color spectrum, can improve the overall LCD color gamut. In addition, the LEDs are Hg free, they provide higher brightness to size ratio, have increased longevity, and can be incorporated in a more robust design. The key issue in introducing LEDs is in finding an efficient way for homogenously spread the LED light over the backlighting panel. Such types of backlight assemblies are disclosed, for example, U.S. Pat. Nos. 6,608,614, 6,930,737, and in U.S. Patent Application Nos. 20040264911, 20050073495 and 20050117320. However, this technique, similarly to CCFL, has an intrinsic power loss of two thirds of the total power due to the use of RGB filters in the LCD panel.
FIG. 42b schematically illustrates another conventional backlighting technique designed to overcome the intrinsic power loss discussed above. In this technique, the colors are separated (instead of being filtered) by prism positioned behind the LCD sub-pixels. Such types of backlight assemblies are disclosed, for example, in U.S. Pat. Nos. 5,748,828, 6,104,446 and in references included therein. This technique, however, suffer from bulkiness and low efficiency due to the bulky optic involved.
FIG. 42c schematically illustrates an additional conventional backlighting technique designed to overcome the intrinsic power loss. In this technique, contrarily to the techniques described above, the colors are guided separately to their destined column of sub-pixels rather than being mixed to white light. Red, green and blue LEDs are coupled to separate optical fibers. The optical fibers illuminate the positions of the red, green and blue pixels of the LCD. The LEDs are constantly on and there is no color filtering.
Such types of backlight assemblies are disclosed, for example, in U.S. Pat. No. 6,768,525 and partially also by U.S. Pat. Nos. 6,104,371 and 6,288,700. This technique, however, is difficult to implement because it requires severe fiber treating and it does not provide solution to the problem of addressing the transmitted RGB lights to the color filters array without crosstalk.
Furthermore, this technique can only provide limited homogeneity in light distribution. For example, in U.S. Pat. No. 6,104,371 to Wang et al. the optical fibers are coupled to RGB light sources and are placed in a sequential parallel order within a panel. Output light uniformity is achieved by placing perpendicular reflecting wedges of increasing height along the fibers, to effect increased reflection which compensates the decrease in optical power along the fiber. However, Wang et al. fail to provide light uniformity at the sub-pixel level. Furthermore, since Wang et al. use a stack of 3×N fibers, where N is a large number, all the RGB colors are mixed at the output.
In U.S. Pat. No. 6,288,700 to Mori, cylindrical waveguides, coupled to RGB sources, are divided to smaller parallel waveguides provided with holes for coupling out the light. The holes are arranged in an addressable arrangement. Such backlighting configuration, however, result in poor performances due to the low efficiency characterizing the coupling of light out of a waveguide through holes. Furthermore, since Mori guides all RGB colors in the same waveguide, there is no separation of colors at the sub-pixels level. An additional drawback of Mori's technique is the lack of uniformity in light scattering or light distribution among the parallel waveguides.
In U.S. Pat. No. 6,768,525 Paolini et al., fibers coupled to RGB light sources are placed parallel in a sequential order and scatter light along their length. The spacing between fibers and the scattering points along each fiber are compatible with the spacing between the sub-pixels of the LCD panel. However, while coupling each color to a separate waveguide, Paolini et al. do not provide any practical technique for achieving a sufficiently accurate arrangement in which different colors arrive at different sub-pixels with minimal mixing. It is recognized that since the coupling of the light out of the waveguide is by scattering, crosstalk between neighbor colors is unavoidable and the uniformity of light at the sub-pixel level for each color is limited. Paolini et al. further disclose a configuration in which the one layer of parallel fibers is replaced by three layers of parallel bulky diffusive waveguides, one for each color. The diffusive waveguides are manufactured with scattering notches. The spaces between the scattering notches are compatible with the spaces between the pixels and the spaces between the bulky waveguides are compatible with the spaces between the sub-pixels.
However, since the parallel diffusive waveguides of Paolini et al. must have a large aspect ratio (narrower than their thickness) and isolated from one another, such configuration has very poor efficiency and uniformity. The reason being that it is difficult to fabricate such waveguide with large aspect ratio and it is difficult to produce large number of diffusive waveguides (one diffusive waveguide for each sub-pixel of the LCD panel) without compromising the optical isolation there amongst.
Although diffusive optical fibers or waveguides are known for backlighting applications, see, e.g., U.S. Pat. Nos. 6,714,185, 6,874,925, 6,910,783, 4,573,766, 5,857,761, 6,072,551, 6,611,303, 6,6714,52 and 6,079,838, such diffusive devices are typically wide and bulky and are mainly coupled to an additional diffuser layer positioned behind the LCD panel, such that there is no direct coupling between the diffusive devices and the pixels or sub-pixels of the LCD panel.
There is thus a widely recognized need for, and it would be highly advantageous to have a device and method for optical resizing and/or providing backlight illumination, devoid of the above limitations.